Uniform excitation of laser gas is of particular importance in the case of molecular gas lasers such as CO2 and CO lasers where over pumping can lead to localized degradation of optical gain in the gas. In addition, electrically pumped gas lasers in general may suffer from instabilities that form under high pulse energy conditions. Electrical discharge instabilities may lead to intense arc discharges which may damage the laser electrodes or at the very least render the optical quality of the gas discharge gain medium useless for producing a high mode quality laser beam. High initial gas discharge uniformity may be used in pulsed, high energy, gas lasers to increase the amount of energy that may be deposited into the gas before the inevitable onset of gas discharge instabilities.
Traditionally, gas lasers have been operated in continuous wave (cw) mode at low gas pressures (about 10 to 100 torr) or as pulsed lasers at high gas pressures (about 300 to 760 torr). At low gas pressures, gas lasers typically have small transverse gas discharge dimensions (about 1 to 4 mm) to produce some degree of discharge uniformity by relying on high rates of ambipolar diffusion in the laser plasma. In addition, extra helium is added to the gas mixtures of low pressure gas lasers to improve discharge uniformity by further enhancing ambipolar diffusion. At high gas pressures, gas lasers usually have transverse discharge dimensions that are too large to allow ambipolar diffusion to be practical. High pressure gas lasers have traditionally used specially profiled electrodes to achieve very good uniform electric field conditions where the gas discharges occur.
Profiled electrodes typically utilize a central region with a flat, parallel, electrode geometry in conjunction with profiled electrode regions chosen to gradually reduce the electric field strength on both sides of the central region while introducing only a minimal amount of electric field distortion in the central region. The gas discharge in a profiled electrode assembly is usually confined to the central region and will have either a square or rectangular cross-section. Unfortunately, the lowest order optical mode of a laser will most likely have a cross-section that is either circular or elliptical and is not a good match for the discharge cross-section of a profiled electrode assembly. About 20% of the energy deposited into the gas discharge of the profiled electrode assembly will not be in the optical cavity of the laser and will be wasted.
Rather than flat electrodes which waste energy, curved electrodes can be used around a cylindrical cross-section. The resulting electric field will fill the optical mode cross-section but unfortunately will be non-uniform. At high gas pressure the RF current flowing through the laser gas of the curved electrode assembly will be focused on both sides of the optical mode cross-section and largely bypass the gas in the center of the electrode assembly. The non-uniformity of the field will also reduce the efficiency of the laser assembly.